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    box-sizing，盒子大小<br>
    值：content-box:内容盒子，设置的宽高是内容的宽高，内边距和边框在盒子之外绘制<br>
    border-box:边框盒子，设置的宽高是大盒子的宽高，内边距和边框在大盒子内绘制<br>
    inherit:继承父元素的box-sizing属性的值<br>
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